Autosomal dominant polycystic kidney disease (ADPKD) is the most common form of inherited renal failure and is caused by mutations two genes, PKD1 and PKD2. Based on the association of aneurysms and ADPKD it has been speculated that polycystins (PKD proteins) play an important role in maintaining blood vessel integrity. An endothelial cell defect has been implicated in this phenotype since Pkd null animals exhibit edema and hemorrhage. Despite the relevance of this problem, there is little known about the role that polycystins play in endothelial cells. We recently used Cre-Lox technology to induce an endothelial cell specific knock out of Pkd1 or Pkd2 in mice. We found that selective inactivation of either gene in endothelial cells yielded a subset of the vascular defects described in Pkd null embryos including ~30-40% perinatal lethality, occasional hemorrhage, polyhydramnios and placental abnormalities. Remarkably, these embryos lacked edema, which is a universal feature of all targeted Pkd null alleles. These studies demonstrated a functional role for polycystins in the endothelial cell compartment and prompted us to hypothesize that loss of functional polycystins results in dysregulation of endothelial cell related signaling pathways. n this application, we propose three specific aims that seek to develop a mechanistic understanding of the functional role that polycystins play in endothelial cells at both the cellula and whole animal level. In Aim1 we will use Pkd null and conditional alleles to explore the basis of edema formation in Pkd1/Pkd2 mutant animals. Aim 2 is based on our observation that polycystin depleted endothelial cells exhibit defective cell migration. We will conduct an in depth characterization of this phenotype using time-lapse imaging and biochemical methods. In addition we will use novel cellular biosensors to probe well-defined signaling pathways implicated in cell migration. In Aim 3 we will employ comprehensive microarray studies of endothelial cells isolated from embryos at different gestational time points to infer the regulator networks that are disrupted in Pkd mutant endothelial cells in vivo. At the conclusion of these studies, we will have gained novel insights into how polycystins fit into the context of vascular development. The hope is that these fundamental paradigms will have broader implications for defining polycystin function, a subject that remains vaguely understood despite many years of intense investigation.